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912 Engine settings

in General Tech Discussion 10 years ago
The Service Letter SL-912-016 gives interesting indications to avoid the risk of overloading the engine.
Among them there is one that I do not understand.
The point 3 of par. 3.1.2) Performance recommendations says that during continuous running the RPM should not be lower than 5200 when in wide open throttle (WOT) conditions.

Flying at 3500 ft I realized to be in WOT conditions getting 26.5 MAP (I own a 80HP 912).
My question is: should I follow the SL indication and set 5200 RPM with my variable pitch prop, being in WOT conditions, or should I follow the Operating Manual and set 4800 RPM?

In both cases I am in WOT conditions. In the case of 4800 RPM I would get about 65% of the power, while at 5200 I get about 75%.
Should the Service Letter setting prevail on the Operator Manual as the latter seems to imply an overloading condition?
I am really confused.
Not to mention the case of fixed pitch prop: how can we set 5200 rpm in WOT conditions as we cannot change the pitch?
Many thanks
Zeb

  • Re: 912 Engine settings

    by » 10 years ago

    Performance recommendations says that during continuous running the RPM should not be lower than 5200 when in wide open throttle (WOT) conditions.
    I think it's called "low speed loading" or "lugging", like driving your car at low speed in a high gear.....not good for an engine designed to rev at high speed.

    In my country, the Aircraft manual overrides the engine manual, however rotax engines love to rev. Obviously at the restricted RPM, you are not getting "potentially" max power.
    I would contact your aircraft manufacturer for clarification. There may be reasons for the restricted power setting.

    Not to mention the case of fixed pitch prop: how can we set 5200 rpm in WOT conditions as we cannot change the pitch?
    Change the prop....
      
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  • Re: 912 Engine settings

    by » 10 years ago

    Rhett is correct. If you can only get 5200 wide open throttle (WOT) in flat and level flight you are over stressing the engine. You could crack the crankcase on top in engines pre mid 2006. That's like driving a stick shift car in 3rd gear at 5-10 mph. You want the right rpm for that engine. It was designed with specific run parameters. Stepping outside those parameters without knowing all the consequences can cause extra maint. and cost to the owner.

    Read this article in the blog section,

    http://www.rotax-owner.com/rotax-blog/item/9-understanding-the-ground-adjustable-prop


    Your engine needs to get a minimum of 5500 WOT in flat and level flight. You would be better off setting a prop up to get around 5600-5700 rpm WOT and then throttle back to a cruise rpm some where between 5100-5400 for cruise. RPM will certainly vary if you are sight seeing, taking off or landing.

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell

    Thank you said by: zeb carris
      
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  • Re: 912 Engine settings

    by » 10 years ago

    maybe I was not clear.
    I have an inflight variable pitch propeller and I can change the pitch as I want.
    My question was: should I always set more than 5200 rpm when flying if I am in WOT conditions?
    This conditions (WOT) takes place every time I fly above about 3500 ft and I do not have a turbo charged engine.
    Thanks
      
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  • Re: 912 Engine settings

    by » 10 years ago

    There may be some varying answers here and not a hard and set number. Some of this is just personal preference, but there is a range to stay in. After take off and prop set for cruise I would have my prop setup to get around 5650 at WOT then pull back the throttle to 5200-5400. Some may say set the prop to get WOT at 5500 rpm and just stay there at WOT. For me that just burns extra fuel without a good return on speed and distance. Some will cruise around 5000-5200 rpm with a WOT of 5000+ rpm. You have some leeway here and some personal preferences to figure out. Some of this may depend on what aircraft, its fuselage and VNE it is set with.

    One thing you do not want to do is set the prop so it can only get to 5200 rpm at WOT and fly there. Your prop pitch should be set to get at least 5500+ rpm then throttle back to where you want to be.

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell

      
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  • Re: 912 Engine settings

    by » 10 years ago

    Thanks Roger,
    As I said I can change in flight the pitch to get whatever (to a certain extent) rpms I want.
    I have been cruising WOT, setting 5000 rpms usually but now, because of the Service Letter SL-912-016, I am in doubt if doing so I am overstressing the engine as I should set the pitch to get at leat 5200 or more rpms.
      
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